[Q] Somethings Wrong with my S4

Hi guys, I never posted here but am trying to find some help. I've had my S4 for quite a while now, and I just factory reset everything because I kept getting a "Unfortunately, the process android.process.acore has stopped." message that wouldn't go away or let me use my phone without an interruption literally every 2 seconds. But before I even had that problem, there were worse problems as well. There are two things that occur frequently:
1. Sometimes, when I tap on an icon, the indicator light shows up (a glow around the icon), but the app itself does not open. Then, if i tap on it repeatedly, nothing happens. I can temporarily fix this problem by opening up the recent/opened apps list (holding home button) and clearing everything (or clicking on that app if it had been opened). Once cleared, I can tap it again and it would open. As you might see, this can get very annoying and troublesome after doing it so many times. Is there anyway I can permanently fix this so that I don't need to keep clearing the open apps list?
2. Sometimes, when I unlock my screen, exit an app, press the home button, or clear the opened app list (as mentioned above), my wallpaper changes to an app. The app is usually random and sometimes is not one that I had opened. For example, the background of my messaging app or Astro File Manager would show up even if I had not recently opened it at all. I can also temporarily fix this by clearing the recent/opened apps list (as mentioned in #1). After clearing this, the wallpaper resets. Is there any way I can fix this permanently as well?
3. Snapchat and VSCO Cam constantly "stop unexpectedly". I don't know if this is a firmware or incompatibility issue, but it usually works fine. This message pops up once in a while and isn't too burdensome, but is still quite annoying.
Thank you for all the help in advance. I am trying to get the most out of my phone and appreciate any help.

Are you running stock or custom and has anything changed since you factory reset?

Sent from my SGH-M919 using Tapatalk

It's stock right now. Not rooted or anything at all. I didn't really change much of the device internally, only downloaded a couple of apps, but those have been working fine since the beginning. It is running Kit Kat, 4.4.2. I am currently using Nova Launcher as my home screen replacement.

XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality.Are you a developer?